Introduction to Naval Weapons Engineering

Underwater Weapons

Figure 1. Minesweeper detonates
a mine.

Underwater Explosions

Explosive detonations which occur underwater create shock waves in a similar manner to explosions in air. Due to the elastic properties of water, the shock wave tends to be of shorter duration, but with a proportionally larger peak overpressure. The energy in the underwater shock wave attenuates very quickly with range. Therefore, the shock wave from an underwater explosion does not cause the same level of damage as one would expect from studying explosions in air. This is not to say that there are no effects from underwater shock waves.

Targets exposed to underwater shock waves will be subject to diffraction and drag loading just like in air. There will be very little translation of targets because the shock wave is of short duration, and objects in motion will quickly be brought to rest by drag in the water. Diffraction loading applies, but targets that are in the water tend to be well-designed to withstand the type of loading that occurs. Therefore, very little damage is done directly by these mechanisms. Considerable damage can still occur due to internal equipment failure and missile hazards (things thrown through the air inside the ship). To be the most effective, underwater warheads utilize the interaction of the steam void created by the explosion with the hull of the target. There are two distinct cases: surface and submerged bodies.

When a warhead is detonated at close range beneath a ship, the steam void initially lifts the ship upwards from the middle. This tends to weaken the ship's keel. After the steam void has reached its maximum volume the surrounding water pressure will collapse it. The ship then falls into the void, still supported on its ends. The keel will then break under the ship's own weight. The compression of the steam void will raise the temperature and the bubble will oscillate a few times. The ship may be destroyed during the subsequent oscillations if it manages to survive the first.

Figure 2. Beneath keel explosion.

When used against a submerged target, the greater ambient pressure and the buoyancy of the surrounding water preclude effective uses of this technique. However the interaction of the steam void and the submerged hull can be exploited to severely damage or potentially breach the submarine's pressure hull. The process begins with a close range underwater explosion. If the steam void contacts the hull it will attach itself. In this location, the oscillation of the steam void will cause a cyclic stress to the submerged hull and weaken or breach it. If a submarine's pressure hull is compromised it is unlikely to survive.

Figure 3. Explosion near submarine
hull.

Depth charges are underwater bombs. They are set to detonate at a prescribed depth. They must be dropped directly on the submarine to be effective. The primary goal in their use is not to sink the submarine, but rather to take it out of action. The ambient noise created by their use will generally deny the submarine the use of sonar. Furthermore, the shock wave may disable internal equipment that could take the submarine out of action. For example, the shock wave may damage torpedo launching equipment or vital propulsion related machinery. Depth charges are typically either thrown over the side, shot out of cannons, or are rocket-propelled. All methods have limited range and accuracy.

Torpedoes

Torpedoes are self-contained weapon systems. At the low end of sophistication, they are straight-running underwater bombs. At the high end, they have active and passive sonar seekers, and can be guided remotely or send telemetry information over a thin wire back to the launch platform. A typical torpedo looks like this:

Figure 4. Lightweight torpedo
being launched from surface ship.

The torpedo sub-sections (from forward to aft) are:

1.) Nose section. This section contains the acoustic tracking system. It also houses the electronic guidance computer.

2.) Warhead section. This section contains the target sensing mechanism. Usually torpedo fuses detect either impact or the magnetic field of the target (see the discussion on mines for magnetic influence sensing). The warhead also contains the main explosive charge. Modern lightweight torpedoes (dropped from aircraft) carry about 100 lbs. of explosive and heavyweight torpedoes pack in excess of 1000 lbs.

3.) Propulsion section. This section contains either electric motor and battery or combustion engine and fuel. The fastest torpedoes use combustion engines. In this case, the performance is limited by depth due to the back-pressure the engine exhaust must work against. Modern torpedoes can travel in excess of sixty nm/hr (knots).

4.) Tail section. This extreme after section contains the control surfaces and propeller.

Sea Mines

Mine warfare continues to be one of the most effective means to deny use of a particular area to the enemy's ships and submarines. Mines can be inexpensive and therefore numerous.

Mines can detect the target's presence through three methods: magnetic, pressure, and acoustic influence. Many mines can use all three in any combination. Magnetic influence mines sense the permanent magnetic field causes by the iron in the ship's hull.

Figure 5. Magnetic influence
sensing.

The magnetic field of a ship can be controlled by periodically degaussing the ship. However, this is a time consuming process which involves wrapping the entire hull in large cables and applying a magnetic field for several hours. Most ships therefore have some detectable magnetic field and are vulnerable to magnetic influence mines. Only ships made out of wood or fiberglass are considered immune. Furthermore, the mine can be made to differentiate different types of targets on the basis of their magnetic "signature."

Mines can also detect targets by the wake they create in the water. As the ship moves through the water, a pressure wave is formed which is visible as a wake on the surface. The same thing occurs underwater but is not visible. The pressure wave can be detected by a mine, however. The faster the ship is moving, the greater the effect.

Figure 6. Pressure influence
sensing.

Lastly, the mine may contain sonar equipment to detect the acoustic "signature" of the ship. This too allows the mine to distinguish between many different types of targets.

Figure 7. Acoustic influence
sensing.

There are two main types of sea mines in use: bottom and moored. Bottom mines rest on the ocean floor. They can be deployed from aircraft, submarines or ships, although deployment from surface ships is rare. In shallow water they are effective against surface ships and in deep water against submarines.

Figure 8. Aircraft deployed bottom
mine.

A moored mine has two parts: an anchor and the mine case. Once deployed (either by aircraft or submarine), the mine case leaves the anchor and deploys to a preset depth. The anchor keeps the mine from drifting.

Figure 9. Submarine deployment
of mines.

The submarine deploys the mine by launching it out of a torpedo tube. Some mines have a limited capacity to travel on their own ("swim") and are used to deploy mines into regions where it is too shallow or dangerous for the submarine to operate. Other mines simply drop to the ocean floor and deploy.

Another variation is the "captor" mine, which is a bottom mine that releases a lightweight torpedo when the target is detected. The torpedo increases the range of the mine. When the torpedo is deployed it begins a circular search pattern using active sonar to look for the target.

Figure 10. Captor mine.

Mines come in a variety of warhead sizes, from a few hundred to a few thousand lbs. of explosive. It should also be noted that drift mines, that is, mines which float freely in the current, are not discussed since their use is prohibited by international law.

Mine Clearing

Removing sea mines is a difficult and dangerous mission. There are three main methods:

Cable cutting. This method is used to clear moored mines. A sled with a cable cutting mechanism is towed through the mine field by a helicopter. The sled cuts the mine case free from their anchors, which are then collected by the mine sweeper. The mine may be detonated by EOD personnel (by placing an explosive charge on the mine) or sunk by gunfire (shooting the mine may cause it to detonate, but not in every case).

Figure 11. Mechanical mine sweeping
sleds.

Detonation by towed sled. The sled is towed by a helicopter as in the first case. This sled is designed to simulate the magnetic, acoustic and pressure signatures of the expected target in order to detonate the mine. The sled is generally of robust construction so that is not destroyed in the process. May not be effective against sophisticated mines which are looking for specific signatures in combination or have delay features which count a specific number of targets before detonation.

Remotely operated robotic vehicles. This process resembles conventional bomb disarming. The robot vehicle has cameras which broadcast video to a controller. The robot may be used to cut the mine free, disarm the mine, or disrupt it. Disrupting is the use of small amount of explosive to separate the fuse from the explosive charge.

Figure 12. Remotely operated
robotic mine clearance device.